ACTA NATURALIA ISLANDICA

ISSUED BY THE ICELANDIC MUSEUM OF NATURAL HISTORY (NATTORUFRA".BISTOFNUN ISLANDS)

The :Museum has published two volumes of Acta N aturalia Islandica in the period 1946-1971, altogether 20 issues. From 1972 each paper will appear under its own serial number, starting with no. 21.

ACTA NATURALIA ISLANDICA is a series of original articles dealing with botany, geology and zoology of Iceland.

ACTA NATURALIA ISLANDICA WIll be published preferably in English, and will appear at irregular intervals.

ACTA NATURALIA ISLANDICA may be obtained: 1) on basis of institutional exchange from Museum of Natural History, P. O. Box 5320, Reykjavik, Iceland. 2) by purchase (charges including mailing costs) from Snaebjorn Jonsson, The English Bookshop, Hafnarstraeti 4 & 9, Reykjavik, Iceland. Distribution and habitat preferences of some intertidal amphipods in Iceland

AGNAR INGOLFSSON Institute of Biology, University of Iceland, Reykjavik.

Abstract. A total of 801 samples of amphipods from 71 sampling stations on all coasts of Iceland were analysed. Results are presented for eleven species belonging to the families Gammaridae and Tali­ tridae. Three species have a limited distribution around the coasts of Iceland, while the remaining species are found more or less commonly on all coasts where suitable shores exist. The distributional patterns observed were compared to those shown by the same species on the western and eastern coasts of the Atlantic. It is concluded that temperature is an important factor controlling the distribution of these amphipods. Habitat partitioning was analysed by use of 41 habitat categories under eleven main headings. Each of the eleven species was found to have a habitat distribution different from that of the other species, with the possible exception of one species pair. Spatially successional series of species could be distinguished along environmental gradients found from sheltered shores with luxuriant fucoid vegetation to exposed shores without fucoids, from high to low tidal levels on rocky seashores, from upper to lower reaches of estuaries and from high-salinity to low-salinity ponds and lagoons. It is probable that habitat partitioning among the species results from differences in adaptations to several interacting physical factors, of which salinity, humidity, temperature, and oxygen level can be reasonably identified. Interspecific competition may reduce habitat overlap among species but the data are inconclusive. 2 AGNAR INGOLFSSON

TABLE OF CONTENTS

Introduction 3 Methods 3 Collection of data 3 Analysis of data 4 Results 6 Gammarus oceanicus SegerstriUe 6 Gammarus locusta (Linne) 8 Gammarus setosus Dementieva 8 Gammarus zaddachi Sexton 9 Gammarus duebeni Lilljeborg 9 Marinogammarus marinus (Leach) 15 Marinogammarus obtusatus (Dahl) 15 Marinogammarus stoerensis (Reid) 17 Marinogamm~rusfinmarchicus (Dahl) 17 H yale nilssoni(Rathke) 18 Orchestia gammarellus (Pallas) 18 Association of species 19 Discussion 20 Distribution 20 Habitat preferences 23 Acknowledgements 25 References 25 INTERTIDAL AMPHIPODS IN ICELAND 3

INTRODUCTION spring-tides along transects worked down seashores and along estuaries. Generally 5 - 7 Intertidal amphipods of the families Gam­ samples were taken along each transect from as maridae and Talitridae in the North Atlantic high up as amphipods were found down to the region have been the subject of many ecological sublittoral. Additional samples were obtained studies in recent years (e.g. Goo d h art 1941, from a variety of habitats whenever these were Spooner 1947, Jones 1948, Steen 1951, encountered. The zonation pattern of each tran­ Den Hartog 1963, 1964, Vader 1965, sect was described with reference to dominant Rygg 1972, Dennert 1973, Steele and organisms, and on each sampling site the sub­ Steele 1975c, Van Maren 1975). Habitat strate was described, cover of species of algae partitioning among species has been dealt with in estimated, presence of other animals noted, and many of these studies, although there is a lack of where appropriate, temperature and salinity quantitative data on this aspect, and general measured, the latter by use of a hydrometer with statements are the rule. The present study was an accuracy of 0.2%0. The size of the sampling undertaken in an attempt to place habitat selec­ sites varied, but was almost always less than 5 m 2• tion of these amphipods on a more quantitative The area searched at each sampling site was basis than had been done previously. There is in reasonably homogenous. A number- of samples addition a unique assemblage of species found on from southwestern Iceland were obtained from Icelandic seashores, the study of which could be sampling sites 800 cm2 in area so that densities expected to yield p~rticularily interesting infor­ could be determined, but otherwise the aim was mation on niche partitioning among related to obtain at least 100 amphipods in each sample, sympatric species. but.this was frequently not possible. Tidal levels A second aim of this study was to delimit the of many sampling sites in southwestern Iceland range of the intertidal species. As the species in were measured with surveying instruments and question are largely confined to the intertidal with reference to the tidal gauge of Reykjavik zone and therefore have an almost linear dis­ harbor. The amphipods were picked up from tribution it was thought possible to delimit the under stones or between algae with forceps. The range of each species rather accurately and thus algae were shaken in order to dislodge the facilitate a consideration of environmental fac­ animals. Sampling from under water was aided tors controlling distribution. Most studies of dis­ by use of hand nets (mesh-size about 1.5 mm) and tributional patterns of marine animals around on tidal flats the substrate down to a depth of 10 Iceland have been marred by a very irregular cm was sieved (mesh-size 1.5 mm). Almost all sampling effort and there was clearly a need for a samples were obtained from May to August in study based on samples distributed evenly 1974 and 1975. around the coasts of Iceland. Additional unpublished data have been used when appropriate, but separately from those of the general survey. The relevant information is METHODS contained in a number of reports compiled by the author and other staff members and students at Collection of data the Institute of Biology, University of Iceland. Amphipod samples were obtained from all coasts of Iceland. Samples were taken during low 4 AGNAR INGOLFSSON

Ana'lysis of data category (10). The rocky seashore is divided into In analysing habitat preferences 41 habitat four types, habitat categories 5 - 8, often referred categories under 11 main headings were esta­ to hereafter as shore types 5 - 8. The distinguish­ blished. The categories were defined prior to ing feature of these types is the species composi­ sampling but somewhat revised after sampling tion of fucoid algae, and they represent a series was completed. Definitions are given in Table 1. from sheltered shores with stable substrate (type 5) Habitats of categories 1-4 are characterized by to exposed shores where considerable movement low and often fluctuating salinities. The lagoons of substrate occurs (type 8). Tidal flats (habitat and coastal ponds (habitat category 1) are of category 10) are of several different types dep­ several different types, but can be roughly ending on substrate and fresh-water influence, grouped into two groups, 1.1 and 1.2, the former but as few samples were obtained from this receIvmg salt water through underground habitat, the category was not subdivided further. seepage from the sea or via sea-spray, the latter A few samples that could not be placed in a connecting with the sea through a narrow chan­ habitat category as defined were placed in cate­ nel. Estuaries (habitat category 3) in Iceland are gory 12 (undefined). usually very short, and pronounced salinity The amphipods of each sample were identified changes are confined to a small area around the and counted under a dissecting microscope. mouth of the river. The term esturary will here be Descriptions and keys by S e g e r s t r a I e used to include the river channel itself traversing (1947), Sexton (1942), Sexton and the intertidal region, as well as the intertidal area Spooner (1940), Stephensen (1928, to each side of the channel if there are no tidal 1935-1942) and S t 0 c k (1967) were the chief flats, while tidal flats are placed in a separate aids used in identification together with a

Table 1

Designations and definitions of habitat categories, and number of samples examined from each category from southern + western and from northern + eastern Iceland.

Number of samples S + W N + E 1. Lagoons and coastal ponds 31 31 1.1. Little or no tidal change, salinities usually lower than 12%0 1.1.1. Above water level 7 3 1.1.2. Below water level 11 19 1.2. Tidal range noticable, salinities fluctuating but rarely exceeding 25% 1.2.1. Upper shore 3 4 1.2.2. Lower shore 3 5 1.2.3. Below water level at low tide 7 0 2. Tidepools 6 18 2.1. Upper shore, above fucoids when these present 7 2.2. Middle shore, between uppermost fucoids and uppermost Fucus dis­ tiehus L. when present 4 9 2.3, Lower shore, below uppermost F. distichus when this species present 1 2 3. Estuaries 11 73 3.1. Upper tidal reaches, in river channel 4 22 3.2. Upper tidal reaches, above river channel 2 11 3.3. Lower tidal reaches, in river channel 2 21 3.4. Lower tidal reaches, above river channel 1 19 INTERTIDAL AMPHIPODS IN ICELAND 5

Number of samples S + W N + E 4. Small streams traversing shore 5 13 4.1. Upper shore, above fucoids when these present 3 6 4.2. Middle shore, between uppermost fucoids and uppermost F. distichus when present 2 4 4.3. Lower shore, below uppermost F. distichus when this species present 0 3

5. General seashore where Ascophyllum nodosum (L.) Le Jol., Fucus vesieulo- sus L., F. distichus (and/or F. serratus L.) present 261 90 5.1. Above fucoids 5.1.1. Shores with Fucus spiralis L. (with or without Pelvetia canaIi- culata (L.) Decne et Thur.) 9 5 5.1.2. Shores without F. spiralis and P. canaliculata 3 7 5.2. P. canaliculata/F. spiralis zone 62 10 5.3. Upper third of F. vesiculosus/A. nodosum zone 57 23 5.4. Middle third of F. vesiculosus/A. nodosum zone 60 14 5.5. Lower third of F. vesiculosus/A. nodosum zone and boundary area with F. distichus (and/or F. serratus) zone 38 14 5.6. Main part of F. distichus (and/or F. serratus) zone 18 16 5.7. Zone between F. distichus (and/or F. serratus) and Laminaria (and/or Alaria) zones 14

6. General seashore where F. vesiculosus, F. distichus (and/or F. serratus) present, A. nodosum absent 18 73 6.1. Above fucoids 6.1.1. Shores with F. spiralis (with or without P. canaliculata) 1 1 6.1.2. Shores without F. spiralis and P. canaliculata 0 12 6.2. P. canalieulata/F. spiralis zone 2 2 6.3. Main part of F. vesiculosus zone 4 21 6.4. Lower third of F. vesieulosus zone and boundary area with F. distichus (and/or F. serratus) zone 5 10 6.5. Main part of F. distichus (and/or F. serratus) zone 6 25 6.6. Zone between F. distichus (and/or F. serratus) and Laminaria (and/or Alaria) zones 0 2 7. General seashore where F. distichus only fucoid present (except F. spiralis occasionally) 9 41 7.1. Above F. distichus 5 11 7.2. Upper third of F. distichus zone 2 14 7.3. Middle third of F. distichus zone 0 4 7.4. Lower third of F. distichus zone 2 11 7.5. Zone between F. distichus and Laminaria (and/or Alaria) zones 0 1 8. Boulder or gravel shores without fucoids 5 38 8.1. Upper third of shore 1 11 8.2. Middle third of shore 2 16 8.3. Lower third of shore 2 11 9. Area immediately below LWS, usually with Laminaria and/or Alaria 0 52 10. Tidal sand- or mudflats 14 3 11. Puccinellia maritima (Huds.) salt marshes 3 1 12. Unclassifiable samples 2 3 6 AGNAR INGOLFSSON mimeographed key by Dr. W. Vader, Tromso Museum, Norway. No attempt was made to identify juveniles smaller than 2.5 mm (excluding antennae). The percentage composition of iden­ tified specimens was calculated for each sample. All samples containing less than 5 amphipods were omitted from analysis. For further analysis the samples were divided ~ornafjordur geographically into two groups on the basis of 64 , distribution and abundance of amphipod species 100 km (Fig. 1): . Vestmannaeyjor 25 2,0 1. Samples from northwestern, northern, " northeastern and eastern Iceland, i.e. the area Figure 1. Outline map of Iceland with chieflocalities north of the bay of Breidafjordur in the west to mentioned in the text. Also shown are boundaries as used in this paper between northern and eastern coasts and including Hornafjordur in the southeast. and southern and western coasts. 2. Samples from southern, southwestern and western Iceland, i.e. the area west of Horna­ fjordur in the southeast to and including the RESULTS northern shores of the bay of Breidafjordur in the west. Altogether 801 samples of anlphipods were The results are presented mostly in the form of obtained from 71 .sampling stations, representing frequencies of occurrance of species in samples some 44.000 identified specimens. Although all from a specific habitat category (or a combi­ identified amphipods were included in the ana­ nation of these). Utilization of percentage com­ position and densities (where available) adds lysis, only the results for 11 intertidal species be­ longing to the families Gammaridae and Tali­ comparatively little further information, as there tridae will be discussed here. Other species ob­ is a fair positive correlation between the tained were rarely abundant III the frequency of a species in a habitat on one hand and the average percentage of amphipods which intertidal zone. They constituted about 13% of it constitutes and its densities in that habitat on the identified amphipods, but about 85% of these the other. There are some exceptions from this, were obtained from the subtidal region (habitat notably in habitats situated at high tidal levels. category 9). Amphipods are scarce at these levels, and a species occurring in many samples from this Gammarus oceanicus Segerstrale region may not at all be common there. This is the most common and widespread of To analyse geographical distribution samples the intertidal amphipods of Iceland, occurring taken within a radius of 5 - 8 km were combined. commonly on all coasts except the south (Fig. 2), This resulted in 71 sampling stations spaced where over wide stretches the only habitats suit­ evenly around the coast (Fig. 2), except that there able for amphipods are low-salinity lagoons. This were few stations along the south coast, where the is the species that S t e p hen sen (1940) refers shore is unsuitable for macroscopic animals over to as Gammarus locusta (Linne) sens. str. (see wide stretches due to surf and moving substrate. S p 0 0 n e r 1951) and is the only species treated Non-parametric statistics have been used in his synopsis of Icelandic amphipods that is so throughout, either the chi-square test or the well represented in collections that individual Mann-Whitney U test. localities are not listed. In the present survey it was found in about half the samples and it con­ stituted almost 30% of the amphipods identified. INTERTIDAL AMPHIPODS IN ICELAND 7

This species occurs under a wide range of con­ self. In the MTL-MHWN tidal interval ditions (Table 2). It is most characteristic of the G. oceanicus becomes the dominant species at moderately exposed rocky shores of types 6 and 7, station 7, but in MLWN-MTL interval this especially the latter (Fig. 4), although the dif­ occurs slightly more upriver, at station 6. It is the ference in frequency on these two types of shores dominant species in the MLWN-MLWS interval does not quite reach significant proportions at all stations where the interval occurs. In the (0.1 >P > 0.05 for whole island). On both types of river channel it replaces G. duebeni at a tidal level shores it is the commonest amphipod present, around MLWN and shows here a broad overlap and although it is also quite common on the more with G. zaddachi. sheltered shores of type 5 and the exposed shores Few samples from tidal flats were obtained of type 8, it is here overshadowed by other species during the survey, but considerable information (Table 2). G. oceanicus is rather evenly distributed on the amphipod fauna of tidal flats has been vertically on general seashore, althoug it does obtained during ecological investigations of show a tendency to increase in abundance several localities in recent years (Table 5). G. oce­ downwards (Fig. 5). On shores of types 5 and 6 anicus is everywhere the most common amphipod, considered together it occurs less often in habitats except on flats characterized by the polychaete 5.3 and 6.3 and those above than in habitats 5.4 Nereis diversicolor, which are more estuarine in and 6.4 and those below (0.05 > P > 0.01 for character than are the other types. whole island). The same trend is also seen on G. oceanicus is found rarely in coastal ponds and shores of type 8 (nonsignificant), while it is hardly lagoons (habitat category 1, Table 2) and in all present on shores of type 7. The species occurs investigated cases salinities reach at least 10%0 on subtidally (habitat category 9) much more a tidal cycle in these habitats, although abundantly than any other species here consid­ measurements as low as 3.2%0 have been ob­ ered (Table 2). tained. In tidepools and small streams (habitat In estuaries (habitat category 3) the species is categories 2 and 4) it is never found above the quite common in the lower reaches, and occurs fucoid zone. here both in the river channel itself (and thus The species is much more frequent on type 5 surrounded by virtually fresh water at low tide) shores in northern and eastern Iceland than on and in the intertidal area on each side of the southern and western coasts (Table 2, P < 0.001), channel (habitats 3.3 and 3.4, Table 3). A special but it appeared equally common in other habitat study was made of the distribution of amphipods groups in the two areas. This is probably due to in the Ellidaar estuary in Reykjavik, south­ type 5 shores of southern and western Iceland western Iceland (Fig. 1), and the data thus ob­ being on the average more sheltered and with tained, which have been kept separate from those more luxuriant fucoid vegetation than those in of the general survey, show well the typical dis­ northern and eastern parts. tributional patterns of amphipods in Icelandic The habitat preferences of the. species outside estuaries. The tidal reaches of the Ellidaar estu­ Iceland appear to be rather similar to what is ary are about 500 m long. Samples were obtained reported here, although quantitative data are along transects run perpendicular to the river scant (B 0 u s fie I d 1973, Bra t t erg ar d channel at 8 stations, station 1 being most 1966, Den n e r t 1973, S e g e r s t r a I e 1959, upriver. The vertical interval between sampling Steele and Steele 1972, Steen 1951, sites along the transects was 50 cm. Samples were T z vet k 0 v a 1968). It is reported from all tidal grouped together into tidal intervals at each levels, but seems to prefer the lower part of the station, as shown in Table 4. G. oceanicus is seen to shore and is frequently encountered subtidally, be common in the lower reaches, replacing even to a considerable depth. It enters the lower G. duebeni seawards at all but the highest reaches of estuaries and has been recorded at tidal levels and also in the river channel it- salinities lower than 1%0 (D e nne r t 1973) 8 AGNAR INGOLFSSON although it is unclear how permanent these con­ more sheltered shores of type 7 (difference in ditions were. frequency on these two types nonsignificant), but here it is overshadowed by G. oceanicus. It is con­ Gammarus locusta(Linne) siderably less common on the still more sheltered shores of types 5 and 6. In low-salinity habitats it As mentioned above, Gammarus locusta (Linne) is found rather sparingly. In estuaries (Table 3) it of S t e p hen sen (1940) was in fact G. oceanicus. occurs especially in the lower reaches, both in the The only previous acceptable record of true G. river channel itself and above it, as does G. oce­ locusta from Iceland is that mentioned by anicus. In two lagoons where the species was found S e g e r s t r al e (1947) from Bakkafjordur, salinities were 10%0 and 11 %0 (single measure­ northeastern Iceland. According to the present ments), while in a third, four measurements done survey the species is rare in the intertidal zone of at various stages of the tidal cycle ranged from Iceland, and there are no records from the east 3-16%0. coast south of Bakkafjordur (Fig. 2). In all it was G. setosus has a tendency to be most common only found in 7 samples, i.e. in one sample from rather high on the shore, usually above the main each of the following habitats (see Table 1): 5.6, occurrance of G. oceanicus. This tendency does not 6.5, 6.6, 8.2 and 8.3, and in two samples from reach significant proportions on shores of type 8, habitat 9. These habitats are all located Iowan but on type 7 shores, the species occurs signifi­ the shore or subtidally. Uniquely the species has cantly more often in samples from habitats been found in abundance subtidally on sandy 7.1+7.2 than from habitats 7.3+7.4+7.5 and muddy bottom at the small inlet of Osar, (0.05> P > 0.01), being especially frequent in the southwestern Iceland (Fig. 1) (H. 7.1 samples. On shores of types 5 and 6 consid­ G u d m u n d s so n, unpup. manuscr.), where ered together (Fig. 5) it is also more frequent in the species also occurs intertidally, but not a samples from habitats 5.3 + 6.3 and those higher single specimen has been found during several up than in samples from habitats 5.4 + 6.4 and additional surveys of sandy or muddy subtidal those lower down (P ~ 0.05). The species is rarely areas in Iceland. encountered subtidally. Outside Iceland the species is found in the In southern and western Iceland G. setosus be­ lowest part of the intertidal as well as subtidally, haves in a rather different fashion from that often on soft bottoms (B rat t ega r d 1966, described above. It is here almost totally lacking G a a d h art 1941, Den H art a g 1964, from the sheltered and semi-exposed shores of Jones 1948, Segerstrale 1959, types 5 - 7. Shores of type 8 are infrequent in this Spa one r 1957, S tee n 1951). area, but it is significant that G. setosus was found to be the dominant species in samples from the Gammarus setosus Dementieva single transect worked on this type of shore. S t e p hen sen (1940) records this species Otherwise the chief habitat of the species in from all coasts of Iceland except the southern southern and western Iceland are coastal ponds coast. In the present survey G. setosus was found to and high-level tidepools (habitats 1.1 and 2.2), be common on shores in northwestern, northern but it is not a common species here. Salinities in and eastern Iceland (Fig. 2). It is also found in these ponds and pools generally ranged from western Iceland south of the northern shore of the 2- 10%0 but they may fluctuate considerably on bay of Breidafjordur, but is much less common a tidal cycle. In one pool where salinities were and more local here. usually below 5%0 measurement of 24%0 was ob­ In northern and eastern Iceland this species is tained on one occasion. G. setosus is usually most common on the exposed boulder shores of accompanied by G. duebeni in these ponds and type 8, and it is the dominant amphipod of these pools, less often by other species. shores (Table 2, Fig. 4). It is also common on the There is scant information on the habitat INTERTiDAL AMPHIPODS IN ICELAND 9 preferences of G. setosus in other areas, although it coastal ponds (habitat category 1). Salinities has frequently been recorded from the intertidal where the species was found ranged from less zone of arctic waters. On the western coast of the than 0.5%0 to about 17%0, and were frequently Atlantic it is found on the general seashore, in found to vary within a tidal cycle. In these tidepools, stream outflows and beach seeps, and habitats the species is usually accompanied by G. in brackish regions of brooks and rivers down to duebeni, but unlike that species it is almost con­ salinities of about 3%0 (B 0 use fie I d 1973, fined to regions that are continuously submerged. V. J. S tee I e and D.H.S tee I e 1970). South G. zaddachi is, however,'absent from many small of Newfoundland the species is only found in the coastal ponds where G. duebeni is present, outlets of cool fresh-water streams in summer although salinities were frequently found to be (S tee I e and S tee I e I.c.). In the White Sea it similar to those of ponds also containing G. zad­ inhabits all horizons of the intertidal area dachi. It does not occur in tidepools. It is found (T z vet k 0 v a 1968), but on the western coast occasionally in small streams running over the of the Atlantic it occurs slightly higher on the shore, but apparently less often than G. duebeni, shore than G. oceanicus (S tee I e and S tee I e but data on this habitat are rather scant. 1974). Den H art 0 g (1964) summarizes what was known of the habitat of this species outside Ice­ Gammarus zaddachi Sexton land, and the present observations are in good This species has not previously been reported agreement with his conclusions. It is often from Iceland, perhaps due to confusion with regarded as a sublittoral species because it is lar­ other species. In the survey it was found to be gely confined to the river channel of esturaries abundant in estuaries and lagoons around the and usually occurs continuously submerged in coast of Iceland (Fig. 2). There are, however, only lagoons, but some workers have also recorded it a few records from northeastern Iceland, prob­ from areas that become exposed at low tide ably due to scarcity of suitable habitats there. (Va d e r 1972, Bra t t e gar d 1966). As in Ice­ The species occurs in a restricted range of low­ land, it is not found in small tidepools (D e n­ salinity habitats (Table 2). It is above all charac­ n e r t 1973). teristic of estuaries, where it is primarily found in the river channel itself, thus remaining sub­ Gammarus duebeni Lilljeborg merged at low tide (habitats 3.1 and 3.3, Table S t e p hen sen (1940) records this species 3). Its distribution in the Ellidaar estuary appears from several localities in Iceland, chiefly from the typical (Table 4). Here it occurs throughout the western coast. In the present survey it was found channel and overlaps broadly with G. duebeni in to be abundant on all coasts of Iceland where the upper reaches and with G. oceanicus in the suitable habitats existed (Fig. 2). lower. It does not always extend as high up the The species prefers low-salinity habitats (Table channel as G. duebeni does, although this is the 2). It is particularily characteristic of coastal case in the Ellidaar estuary, and it is sometimes ponds and lagoons (habitat category 1), occurring found subtidally in front of mouths of large here both where continously submerged as well as rivers. It is found in small numbers in the inter­ in the intertidal area (where tidal changes tidal region on each side of the river channel, present) or just above water level (where there are especially close to the channel. On tidal flats no tidal changes). In the coastal ponds (1.1) characterized by Nereis diversicolor, which are un­ salinities ranged from 0.15-12%0, but where der considerable fresh-water influence, G. zad­ frequently around 2%0. Only small fluctuations dachi is the dominant amphipod, and is here often of salinities were found were repeated accompanied by the amphipod Pseudalibrotus lit­ measurements were made. Salinities fluctuate toralis (Table 5). more in the lagoons (1.2). Three examples of G. zaddachi is also cOII).mon in lagoons and these fluctuations on a single tidal cycle are as 10 AGNAR ING6LFSSON

66

~

G.mm,,", ''''"''"'~

64 / 64 " .GJ 25 20 " 20 15 ~~\)~{~"1'.~J~,-.

~ Gammarus setosus ~?kJ 64 ti:.'"\- "",~H 64 25 20 " "

Gammarus duebeni

.. • 20 " "

Figure 2. Distribution of Gammarlls spp. and of Marinogammarlls marin liS in Iceland according to present survey.• positive sampling stations. Onegative sampling stations. Segerstriile's (1947) record of G. loclIsta is also shown (.). INTERTIDAL AMPHIPODS IN ICELAND 11

" "

25 '0 15 25 '0 15

66 "

Hyale nilssoni

25 '0 15 25 '0

"

Orchestia

,,' 20

Figure 3. Distribution of Marinogamtnarus spp., Hyale nilssoni and Orchestia gammarellus in Iceland according to present survey. Symbols as in Fig. 2. Records from Stephensen (1940) that extent the distributional area of M. finmarchicus and M. stoerensis are also shown (.). 12 AGNAR INGOLFSSON

Table 2

The occurrance of the 9 most common species of intertidal amphipods in samples from southern + western and northern + eastern coasts of Iceland. See Table 1 for definitions of habitat categories. A = percentage of samples in which a species occurs. B = average percentage of amphipods which a species constitutes in those samples where it occurs, when occurring in 5 or more samples.

Average Habitat Number number cate­ of per G. ocean. G. set. G. zadd. G. dueb. M. obtus. M. Ifinm. M. mar. M. stoer. H. nils. gory Area samples sample ABA BA B A BAB A BA BA BAB 1 S+W 31 40 619 32 13 90 92 0 o 13 o o N+E 31 29 6 10 35 62 77 86 0 o o o o 2 S+W 6 27 33 50 o 50 o 33 o o o N+E 18 25 33 80 11 o 56 93 6 o o o 17 3 S+W 11 24 18 0 64 69 64 73 0 o o o o N+E 73 27 27 30 22 46 64 81 40 62 0 3 o 7 30 0 4 S+W 5 15 15 0 40 100 65 0 o o o o N+E 13 21 31 31 8 69 91 0 o o o o 5 S+W 261 35 20 52 1 o 9 55 62 66 19 42 27 45 2 36 34 55 N+E 90 75 56 55 31 24 o 14 75 60 66 23 24 0 14 8 17 19 6 S+W 18 36 67 54 0 o 6 39 58 33 19 28 25 0 39 46 N+E 73 68 73 64 22 15 o 21 72 34 63 8 17 0 18 21 12 18 7 S+W 9 145 89 96 0 o o 11 o 11 o 22 N+E 41 68 85 71 54 29 o 2 12 46 2 o 31 27 0 8 S+W 5 37 40 100 97 o o 20 o o o o N+E 38 82 63 53 68 70 o 8 5 5 o 11 o 9 S+W o N+E 52 117 65 25 8 o o 2 0 0 2 o 10 S+W 14 58 43 65 0 o 14 43 33 36 28 21 o o N+E 3 44 100 0 50 o 33 33 0 o o 11 S+W 3 18 o 0 o 67 o 0 0 o o N+E 1 24 o 0 o 100 o 0 0 o o 12 S+W 2 35 50 0 o o o 0 50 o o N+E 3 63 67 0 o 33 o 0 0 o o

Table 3 follows: 0.4-15%0, 6- 25%0, 3-16%0. The species is often accompanied by G. zaddachi in Percentage of samples from different parts of es­ these habitats, more rarely by other species, but tuaries (habitat group 3) in which species of frequently it is the only species present. G. duebeni amphipods occur. See Table 1 for definitions of was once found in a warm spring a few hundred habitats 3.1. - 3.4. meters from the shore. The temperature was 18.3-19.0 ec, and the water was slightly saline (0.5 -1.0%0). It was the only amphipod present '" 8 and was abundant. There is one previous record '5 ~ .~ Number .0 "0 of this species from a warm spring in Iceland of ~ "0 ] (Schwabe 1936), also situated close to the Habitat samples 0 o o ;:8 ;:8 o shore (temperatures 10- 20 e C). 3.1. 26 56 74 4 7 4 o In small tidepools located above the fucoicl 3.2. 13 85 23 15 15 o o 3.3. 23 13 65 43 30 9 9 zone or at a comparative tidal level (habitat 2.1) 3.4. 20 30 75 45 25 10 o G. duebeni is usually the only amphipod present, INTERTIDAL AMPHIPODS IN ICELAND 13

Table 4

The distribution of amphipods in the Ellidaar estuary, Reykjavik, southwestern Iceland, based on pooled data obtained on 11 October 1972, 27 September 1973 and 3 October 1974. Figures in the main body of the table show percentage composition of pooled samples grouped together according to tidal intervals. Heigh levels are given in m above chart datum

Station

2 3 4 5 6 7 8 Height of water level in river channel at low tide

3.6 3.4 2.1 1.6 1.2 1.1 1.0 0.7

MHWN-MHWS N=O N=50 N=120 N=83 N=36 N=30 N=45 N=O (2.87 - 3.94) Gd 64 Gd 88 Gd 80 Gd 97 Gd 100 Gd 89 Gz 32 Gz 11 Gz 20 Go 3 Gz 9 Go 1 Go 2

MTL-MHWN N=107 N=115 N=120 N=42 N=114 N=41 (2.07 - 2.87) Gd 93 Gd 84 Gd 61 Gd 48 Gd 4 Gz 3 Gz 6 Gz 14 Gz 20 Gz 7 Go 96 Go 97 Go 1 Go 2 Go 19 Go 45 Ms 1

MLWN - MTL N=37 N=63 N=55 N=35 N=60 N=90 (1.30 - 2.07) Gd 92 Gd 73 Gd 53 Gd 3 Go 97 Go 91 Gz 8 Gz 27 Gz 11 Gz 6 Ms 3 Mf 9 Go 36 Go 91

MLWS - MLWN N=37 N=40 N=41 (0.12 - 1.30) Gz 19 Gz 10 Go 95 Go 78 Go 90 Mf 5 Ms 3

River channel N=12 N=69 N=79 N=83 N=82 N=35 N=48 N=29 (low tide) Gz 100 Gd 29 Gd 56 Gd 54 Gd 5 Gz 51 Gz 40 Go 96 Gz 71 Gz 43 Gz 46 Gz 66 Go 49 Go 60 Mf 4 Go 1 Go 29

Abbreviations: Gd = Gammarus duebeni Ms = Marinogammarus stoerensis Gz = G. zaddachi Mf = M. finmarchicus Go = G. oceanieus N = Number of specimens

although occasionally accompanied by G. setosllS. dllebeni becomes scarcer and is replaced by several Salinities of these pools are extremely variable, other species. but they apparently never become hypersaline in In estuaries G. dllebeni is chiefly found in the Iceland. In tidepools further down on the shore G. upper reaches (habitats 3.1 and 3.2, Table 3) and 14 AGNAR INGOLFSSON

Table 5

The dominant amphipods of three different types of tidal flats in Iceland, based on the result of ecological surveys performed by staff and students of the Institute of Biology, University of Iceland. Species are listed in order of dominance, and those species that were common only on some of the tidal flats investigated are placed in parenthesis.

Tidal flats Tidal flats Tidal flats dominated by dominated by dominated by Nereis diversicolor Mytilus edulis L. Arenicola marina (L.) O. F. MUller (4 localities) (6 localities) (3 localities)

Gammarus zaddachi G. oceanicus G. oceanicus Pseudalibrotus (Marinogammarus P. femorata littoralis (Kriiyer) stoerensis) (Anonyx nugax (Phipps» (G.oceanicus) (P. littoralis) (G. duebeni) (Pontoporeia femorata Kriiyer)

its distribution in the Ellidaar estuary appears G. duebeni is not uncommon on the general typical (Table 4). It occurs both in the river seashore, and here it occurs more frequently on channel itself and in the intertidal area to each the more sheltered shores of types 5 + 6 than on side of the channel. In the Ellidaar estuary the shores 7 +8 (0.05 > P> 0.01 for whole island). On species is common in the river channel down to a shores of types 5 and 6 it is virtually confined to tidal level near MLWN. To the side of the chan­ habitats 5.3 and 6.3 and those above (Fig. 5) and nel it is prominent at all stations in the uppermost in this region its frequency is higher in samples tidal interval (MHWN-MHWS), but at lower from immediately above the fucoids (5.1 + 6.1) tidal levels it decreases gradually seawards, and than in samples from the uppermost part of the this decrease starts sooner the lower the tidal fucoid zone (5.2 +5.3 + 6.2 +6.3) (P < 0.001 for level. It is accompanied by G. zaddachi almost whole island), but amphiods are rather scarce at everywhere, but is replaced seawards by G. oce­ the former level. Similar, although nonsignificant anicus at all except the highest tidal levels. The trends are seen on shores of types 7 and 8. species is not found in rivers above the level of The habitat preferences of G. duebeni in Iceland tidal influence. are on the whole similar to what is reported for In small streams traversing the seashore the species in other regions (B 0 U s fie 1d 1973, (habitat category 4) G. duebeni is chiefly found at Bra t t ega r d 1966, Den n e r t 1973, Den upper tidal levels. Above the fucoid zone or at a H art 0 g 1964, Jon e s 1948, Kin n e 1959, comparative tidal level (habitat 4.1) it is usually R Yg g 1972, S e g e r s t r ale 1946, SP 0 0 n e r the only species present. Normally it does not 1957, S tee 1e and S tee 1e 1969, T z vet k 0 v a extend above the Verrucaria-Littorina zone 1968). In Iceland it is not, however, found in fresh (supralittoral fringe) but in one case animals were water considerable distance from the shore as it is

found in a stream about 8 m higher than the frequently in the Faroes (P 0 u 1sen 1928), parts uppermost part of the supralittoral, and in the of the British Isles (H y n e s 1954, S ute 1iff e most extreme case the vertical distance was about 1967,1974) and Brittany (Pinkster et al. 20 m. The horizontal distances were roughly 5 1970) and it is worth mentioning that the fresh­ and 15 m respectively. water species Gammarus pulex (L.) and G. lacustris INTERTIDAL AMPHIPODS IN ICELAND 15

Go infrequent but it has been found occasionally on /.~ tidal flats and in lagoons (habitat categories 10 80 / and 1). Salinities in these lagoons have ranged • Gs from 5-30%0 and fluctuate widely on a tidal ,0 cycle. M~ / • The species has a rather restricted vertical dis- 60 / > •\ 0/ tribution. It is not infrequent in samples from just () above the fucoid zone (habitats 5.1 and 6.1), but Z w reaches maximum frequencies in the uppermost ::J part of the fucoid zone (habitats 5.2, 5.3, 6.2 and o 40 w OMs 6.3), and is clearly much less frequent below this a: o~ "./A (Fig. 5), although present in small numbers down u.. MJ ~ ~ ~ to the sublittoral. M. marinus is the dominant 20 Hn~~/ Gammaridae in the P. canaliculata/F. spiralis zone 1'::::---- (habitats 5.2 and 6.2), but is greatly outnumb- -:~ D~ A ered by M. obtusatus already in habitats 5.3 and ~ _CIJ 6.3. With reference to tidal levels M. marinus can ~----~---~"'~- "* - be said to be common in the MHW-MTL region. S 6 7 8 Elsewhere the species is also found to inhabit SHORE TYPE rather high tidal levels, usually above MTL 00 n e s 1948, Sex ton and S p 0 0 n e r 1940, Figure 4. The frequency of six species of intertidal S p 0 0 n e r 1957). It may extend down to about amphipods in samples from shores of types 5-8 (see Table 1) in northern and eastern Iceland. The species MLWN (G 0 0 d h art 1941) but according to are: Gammarus oceanicus (Go), G. setosus (Gs), Marino­ D en H art 0 g (1964) and Vader (1965) only gammarus obtusatus (Mo), M. finmarchicus (Mf), M. when M. obtusatus is absent. Its upper limits stoerensis (Ms), and Hyale nilssoni (Hn). appear usually to be around MHW. It is sometimes found in streams traversing the shore 00 n e s 1948) and extends some distances into estuaries (D e n H art 0 g 1964, S p 0 0 n e r G. O. Sars have not been found in Iceland, 1957). It has often been recorded from muddy although the latter certainly occurs under similar substrates 00 n e s 1948, Va d e r 1965, Van temperature conditions elsewhere (0 k I and Maren1975). 1969). Marinogammarus obtusatus (Dahl) Marinogammarus marinus (Leach) This is by far the commonest Marinogammarus This species is recorded from only one locality species in Iceland, and S t e p hen sen (1940) by S t e p hen sen (1940), the Vestmannaeyjar records it from several localities that are all in Islands in southwestern Iceland. The present in­ southwestern and western Iceland. In the present vestigation showed it to be confined to the investigation the species was found to be common southwestern and western shores of Iceland, from on all coasts, except for the south coast (Fig. 3), the Vestmannaeyjar Islands in the south to the where lack of suitable habitats is undoubtedly northern shore of the bay of Breidafjordur in the responsible for its absence, as is true of many of north (Fig. 2). It is abundant in suitable habitats the other amphipod species. north to the southern shore of this bay, but much The species is expecially characteristic of the less so further north. sheltered rocky shores of type 5, where it is usu­ The species is most frequent on sheltered shores ally the commonest amphipod (Table 2, Fig. 4). of types 5 and 6 (Table 2). In other habitats it is It also occurs frequently on the somewhat more 16 AGNAR INGOLFSSON

A B S+W C V D 9 U G. duebeni M. marinus G. setosus H. nilssoni

A B N+E LLJ=l C D U Q Q 100%

A B S+W C D Q lb M. stoerensis M. obtusatus M. finmarchicus G. oceanicus

A B N+E C D V 0 Figure 5. The frequency of eight species of intertidal amphipods in samples from different tidal levels on shores of types 5 and 6 in northern and eastern (N+E) and in southern and western (S +W) Iceland. A = habitats 5.1 +6.1. B = habitats 5.2+5.3+6.2+6.3. C = habitats 5.4+5.5+6.4. D = habitats 5.6+5.7+6.5+6.6. See Table 1 for definitions of habitats.

exposed shores of type 6, although significantly 6.4) (Fig. 5). It occurs significantly more often in less so (0.01> P > 0.001 for northern and eastern sample from these habitats, taken as one, than in Iceland). In other habitats it is less common, and samples from above (P < 0.001) and below although it was obtained in a rather high pro­ (P < 0.001) this on shores of types 5 + 6 (for whole portion of the few samples from tidal flats island). In southwestern Iceland, where reference (habitat category 10), other data indicate that it to actual tidal levels can be made, the species is is not a common species in such situations (Table seen to be common at all levels below about 5). It is absent from low-salinity environments MHWN, while it reaches maximum abundanc~ (habitats 1,2.1,3,4). in the MLWN-MTL interval. M. obtusatus is common at all tidal levels in the The habitat preferences of M. obtusatus in other fucoid zone but is rare subtidally. It is also rather regions appear similar. In Newfoundland it is, frequent in samples from immediately above the however, said to occur only on moderately exposed fucoid zone (habitats 5.1 and 6.1), but shores (D. H. S tee 1e and V. J. S tee I e amphipods are scarce here. The species reaches 1970), and generally it does not seem to occur maximum abundance rather low on the shore.-­ higher on the shore than to MTL (G 0 0 d h art around the boundary of the A. nodosum/F. vesicu­ 1941, Den Hart 0 g 1964,J 0 ne s 1948, S ex­ losus and F. distichus zones (habitats 5.4, 5.5 and ton and Spooner 1940, Spooner 1957, 17 INTERTIDAL AMPHIPODS IN ICELAND

Vader 1965, Van Maren 1975). Most shores of type 5 in southern and western Iceland authors stress that it is essentially in intertidal than in the north and east (P < 0.001). This may be species that is rare subtidally. due to an average differences between the two areas in shores classified as type 5 (d. G. oceanicus, Marinogammarus stoerensis (Reid) p.7). S t e p hen sen (1940) records only a single Outside Iceland the species is usually said to be specimen from Iceland, from Hornafjordur in the strictly indertidal and largely confined to areas southeast. The present investigation showed it to on the shore where there is considerable fresh­ occur around the coasts of Iceland (Fig. 3). It is water influence (B 0 u s fie I d 1973, Jon e s however, absent from much of the south coast, 1948, Sexton and Spooner 1940, undoubtedly due to lack of suitable habitats. It S p 0 0 n e r 1957, S tee I e and S tee I e 1975a, seems furthermore to be rather local, and was in V a d e r 1965). Preferred tidal levels are stated to all found at less than half the sampling stations. be between MHWN and MLWN (S ext 0 nand Due to the small size of this species it is less easily Spooner 1940, Vader 1965) and below collected than other Gammaridae species- and MTL (B 0 u s fie I d 1973). It has been found to may therefore be more common than the data penetrate short distances up estuaries (S ext 0 n indicate. and S p 0 0 n e r 1940). These observations are in The species appears to be most common on the fair agreement with the present data, except that moderately exposed shores of type 7 (Table 2, there is little indication that the species shows a Fig. 4). It is significantly less common on shores of preference for regions of the intertidal in Iceland types 5 (P < 0.001 for whole island) and 8 with strong fresh-water influence, and it was (0.05> P > 0.01 for northern and eastern Ice­ never found in fresh-water streams traversing the land), while the difference between shores shore (habitat category 4). of types 6 and 7 is nonsignificant. The data indicate little preference for a particular Marinogammarus finmarchius (Dahl) tidal level, but in northern and eastern Ice­ S t e p hen sen (1940) records this species land it shows a slight but significant tendency from only one locality in Iceland, Djupivogur on to prefer middle levels of shores of types the southeastern coast. In this survey it proved to 5 and 6 considered together (i.e. habitats be rather common on shores on the whole of the 5.2-5.5 and 6.2-6.4) rather than those western coast, from the southwest to the wes­ above and below considered as one group ternmost part of the north coast (Fig. 3). It was (0.01 > P > 0.001) (Fig. 5). The species is also found on the northern and eastern coasts, occasionally met with subtidally, and but was decidedly more local here. This can there are a few records from the lower probably be ascribed to a scarceness on northern reaches of estuaries- (Tables 3 and 4). It has not and eastern coasts of the sheltered shores with been found in other low-salinity habitats. rich fucoid vegetation preferred by this species. Although not recorded from tidal flats in the Its absence from southern shores is undoubtedly general survey, the species was surprisingly found due to lack of suitable habitats there. to be very abundant in beds of mussels (Mytilus The habitat preferem;es of this species in Ice­ edulis) during a survey of tidal flats in Hval­ land appear almost identical to those of M. obtu­ fjordur, southwestern Iceland (unpublished satus but it is a much less common species. It is reports compiled by G. M. Gis I a son and most common on the sheltered shores of type 5, S. S. S nor r a son). The species was not and although not infrequent on shores'of type 6 recorded at all in two additional tidal flats in­ also (Table 2, Fig. 4) it occurs here significantly vestigated, where Mytilus edulis was the dominant less often (0.05 > P > 0.01 for whole island). In animal. other habitats it is much less common, but unlike The species is considerably less frequent on M. obtusatus it has been found in a few samples 18 AGNAR INGOLFSSON from the lower reaches of estuaries (Tables 3 and levels, but shows a clear preference for the upper 4). part of the fucoid zone (Frig. 5). In particular it is M. finmarchicus is found at all t~dal levels on common in the P. canaliculatal F. spiralis zone of shores of types 5 and 6, although it is scarce, as are these shores (habitats 5.2 and 6.2), being more other amphipods, above the fucoid zone (habitats frequent here than above the fucoids (habitats 5.1 and 6.1) but as M. obtusatus it reaches 5.1 +6.1) (P< 0.001 for whol~ island) and im­ maximum abundance rather low on the shore, in mediately below (habitats 5.3 + 6.3) (P < 0.001 habitats 5.4, 5.5 and 6.4 (Fig. 5). Its frequency in for whole island). This preferred level corre­ samples from these habitats is, however, only sponds roughly with the area around MHWN on significantly different from its frequency in these shores. It is not found subtidally. samples from above and below this when samples On both types of shores the species is much from these two levels are pooled (0.05 > P > 0.01 more frequent in samples from southern and for whole island). western Iceland than in those from the north and On the western shores of the Atlantic M. fin­ east (0.01> P >0.001 for type 5 shores, marchicus is reported to occur on the average 0.05> P > 0.01 for type 6 shores). In part this may higher on the shore than M. obtusatus (S tee 1e be so because the algae from many sampling sites and S tee 1e 1975b), but on the European coasts in southern and western Iceland were more tho­ their preferred tidal levels appear to be similar roughly searched for amphipods (in the labora­ (Brattegard 1966, Spooner 1957, tory) than was possible for samples from other T z vet k 0 v a 1968) and the two are often said areas, and it was found that H. nilssoni constituted to occur together. Several authors mention a a much higher percentage of amphipods ob­ tendency for M. finmarchicus to occur in tidepools tained from the algae than of amphipods col­ (Steen 1951,]ones 1948, Spooner 1957, lected by searching the sampling sites after the Den n e r t 1973, B 0 u s fie 1d 1973). B 0 u s­ algae had been removed. In a number of samples fie 1d (1973) even refers to it as being a from southwestern Iceland 70% of the 590 H. dominant species of tidepools at about the level of nilssoni obtained came from the algae, while for MHW in New England. Tidepools at this level M. marinus, for example, the corresponding per­ are in Iceland almost exclusively inhabited by G. centage was only 13% of the total of 129 animals. duebeni, but M. finmarchicus has been found spar­ There was no apparent difference in this respect ingly in tidepools lower on the shore. among the various Gammaridae species. This also shows that the microdistribution of M. H yale nilssoni (Rathke) marinus and H. nilssoni is clearly different, S t e p hen sen (1940) records this species although they occur at similar levels on the same from several localities, all except one (Djupivogur types of shores. in southeastern Iceland) on southwestern coasts. I Habitats reported for the species elsewhere are found the species to be quite common around the on the whole similar (B 0 u s fie 1d 1973, coasts of Iceland (excepting the south coast), C h e v r e u x and Fag e 1925, Goo d h art although somewhat local on northern shores (Fig. 1941, Den H art 0 g 1963, SP 0 0 n e r 1957). 3). It seems to be most abundant in the southwest. H. nilssoni is more difficult to collect than Orchestia gammarellus (Pallas) species of Gammaridae, because of its active be­ This species was recorded from Iceland for the haviour, and may therefore be more common first time by I n g 0 1f s son (1974). It has a very than the records indicate. limited distribution, being confined to the ex­ This species is almost totally confined to shores treme southwest, from the Vestmannaeyjar of types 5 and 6 with rich fucoid vegetation, and Islands to Reykjavik (Fig. 3). The species is quite it is about equally common on both types (Table common within its limited range. 2, Fig. 4). It has been found commonly at all tidal In the survey the species occurred only in 4 INTERTIDAL AMPHIPODS IN ICELAND 19 samples, all taken from the P. canaliculata/F. out the vertical extent of the marsh with a peak in spiralis zone (habitat 5.2) or Puccinellia salt mar­ the middle of the Puccinellia zone. The upper limit shes (habitat 11). In spring of 1975 much of the of the species appeared to the near the level of distributional area of the species was traversed on highest annual high water, considerably above foot for the purpose of mapping the intertidal the highest Puccinellia plants (I n g 0 I f s son region. O. gammarellus was frequently encount­ 1977). ered, specially under stones on Puccinellia salt These observations agree well with those from marshes, which are at a slightly higher level than other regions (e.g. Den H art 0 g 1963). the uppermost P. canaliculata, but it was also found in the P. canaliculata/F. spiralis zone. In Association of species 1976 a survey of salt marsh animals by use of Association of species was tested by inves­ pitfall traps at Galgahraun near Reykjavik tigating by use of chi-square whether any two of showed O. gammarellus to be abundant through- the 9 most common species occurred more, or less

Table 6

Chi-square tests of association among the 9 most common species of intertidal amphiods in northern and eastern Iceland (above diagonal, 436 samples) and in southern and western Iceland (below diagonal, 365 samples). Positive association is indicated by +, negative by , and no association by O. Note: There are no tests involving M. marinus above diagonal and M. stoerensis below diagonal because of scarcity of these species.

;:l'" u

'";:l 'fil'" ;:l :.a :e .... ;:l'" '" u '8 ... oj ~ oj v oj ~ ;:l 'u 'S ;:l ~ '" ~ 0 "0 {J S '" 'C v oj ~ .... oj 0 "0 ;:l ..c 0 ....'" v '" oj tC 0 S .... v U N "0 '" '" 0 :s d d :;8 :;8 :;8 :;8 d d i G. zaddachi +

G. duebeni 0 0

M. finmarchicus 0 +++ 0 0 0

M. obtusatus +++ + 0 +++

M. marinus 0 0 0 0

M. stoerensis +++ +++ +

G. setosus 0 ++ 0 0 +++ 0

G.oceanicus 0 0 0 + 0

H. nilssoni 0 ++ 0

Levels of significance: + or­ 0.05>P>0.01 ++ or-­ 0.01 >p>O.OOl +++ or --- P< 0.001 20 AGNAR INGOLFSSON

often together in samples than predicted by ference in preferred tidal levels. H. nilssoni and M. chance. Separate analyses were performed for marinus, on the other hand, occupy similar levels samples from northern and eastern and for and are therefore positively associated. Finally, G. southern and western Iceland. An analysis of this oceanicus, G. setosus and M. stoerensis form a group type would reveal similarities and dissimilarities as they are all common on moderately exposed in habitat preferences among species indepentently shores. of the subjective definitions of habitat categories. As the samples are not, however, taken randomly in all possible habitats of the amphipods in question, some care is needed in interpreting the results. The results are seen in Table 6. It is possible to DISCUSSION distinguish the following groupings of species, although they are not distinct: Distribution (a) G. zaddachi and G. duebeni Much evidence points to temperature being an (b) M. obtusatus and M. finmarchicus important factor controlling the geographical (c) M. marinus and H. nilssoni distribution of marine animals (e.g. H e d g­ (d) G. oceanicus, G. setosus and M. stoerensis pet h 1957, Br i g g s 1974). All four groups are interlinked by some The sea surface temperature during winter positive associations except groups (a) and (b) around the coast of Iceland is highest off the and groups (a) and (c). All possible combinations south coast, being lower off the west coast while (pairs) of the 9 species occurred at least once, temperatures are similar and lowest off the except that G. zaddachi and M. finmarchicus were northern and eastern coasts (Fig. 6). In summer, never found together in a sample. the temperature is highest off southern and wes­ The results are in general agreement with those tern coasts, lower off the northern and obtained by habitat analysis, and indicate that northeastern shores and lowest off the middle of the habitat categories here used are meaningful the east coast. There is a sharp temperature units with respect to habitat preferences of boundary off the southeast coast, where temper­ amphipods. Clearly G. zaddachi and G. duebeni atures may change for 4-5°C over a distance of form a group because they occur in low-salinity some 90 km (S t e fan s son 1969). The above environments much more frequently than other temperatures are based on data collected some species. In addition G. setosus has in southern and distance from the shore and thus give only a western Iceland withdrawn from most high­ rough indication of the sea temperatures ex­ salinity environments occupied in the north and perienced by intertidal animals. east, and therefore forms a positive association Several authours (e.g. S t e ph ens e n 1940, with G. duebeni in the former but not in the latter Thorson 1941, Einarsson 1948, Mad­ area. M. obtusatus and M. finmarchicus both occur sen 1949) have investigated distributional pat­ primarily on shores of type 5. The majority of the terns of marine animals in Iceland. B rig g s samples (72%) from southern and western Ice­ (1974) summarizes the results as follows (p. 261): land come from this type of shore. Tests of "Examination of data on the local distribution of associations in this area are therefore largely tests both fishes and invertebrates show a definite of associations within this type of shore. In spite pattern. Both the purely boreal species and the of this M. obtusatus and M. finmarchicus remain eurythermic temperate forms tend to be confined positively associated, whereas M. obtusatus and H. to the south and west coasts, the arctic species are nilssom; which are positively associated in samples mainly restricted to the north and east, and the from northern and eastern Iceland, become arctic-boreal animals are generally found all negatively associated here because of the dif- around the island". Consideration of marine INTERTIDAL AMPHIPODS IN ICELAND 21

east coast while on the south coast the difference 1.8 is very small. On the whole air and sea temper­ .. ,~' atures around the coast of Iceland vary in a similar ~~ manner and it would be difficult to separate the 9.9 effects of air and sea temperatures on dis­ 3.5 10.4 tributional patterns of intertidal animals. Air 4.2 10.2 temperatures probablysomewhat reduce the dif­ ference in temperature regimes between the in­ tertidal area of the east coast and that of other coasts, as compared with sublittoral habitats. 6.4 Of the 11 species of amphipods here consid­ 20 10.9 2S 15 ered, three have a limited distribution in Ice­ Figure 6. Mean surface temperatures at selected land. O. gammarellus is confined to the extreme stations around the coast of Iceland in February (up­ southwest, while M. marinus is only found on per figures) and August (lower figures) based on data southwestern and western shores. Both are there­ collected 1949-1966 (Stefansson 1969). fore limited to the part of the Icelandic coasts where temperatures (both air and sea) are on the average highest. One species, G. locusta, seems to algae 06 n s son 1912, A dey 1968, M u n d a be lacking only from the east coast, where sea 1972, 1975) have yielded very similar results. The temperatures are on the average lower than overall correspondence between distributional elsewhere. patterns and the temperature of the sea around While G. setosus is found on all coasts, it is the coasts of Iceland is therefore strong. scarce on the warm southwestern coasts. The Few of the animals considered in the above remaining species are found commonly on all surveys have been mainly intertidal in habits. For coasts where suitable habitats exist. intertidal species both sea and air temperatures All species considered occur in Norway. The can be expected to be important, interacting in a first species to reach its northern limit here is O. complex manner (S 0 u t h ward 1958). In Ice­ gammarellus which has been recorded north to land air temperatures at coastal stations gener­ Vreroy (67°40'N) (Stephensen 1935­ ally decline from south to north (data in 1942). The next is M. marinus for which the Eythorsson and Sigtryggsson 1971) northernmost locality is Havoysund (71°00'N) on both western and eastern coasts. In summer, (W. Va d e r pers. comm.). G. locusta occurs all however, temperatures along the east coast are the way to Vardo at the northeastern extremity of 1-2°C lower than at corresponding latitudes on Norway (0 Ide v i g 1959), and M. stoerensis ex­ the west coast and similar to temperatures of the tends slightly further eastwards into colder north coasts. Based on average temperatures waters to Petsamo (S e g e r s t r 11 I e 1948). The from 1931-1960, July temperatures range from remaining species extend still further eastwards. a little more than 11°C in the extreme south to a H. nilssoni does not appear to reach the White little above 8°C in the north and east, while Sea, while M. finmarchicus and M. obtusatus do so average January temperatures range from about (T z vet k 0 v a 1968). Both G. duebeni and G. 1-2°C in the extreme south to a little less than zaddachi reach still further eastwards. at least to +2°C in the north. Sea temperatures exceed air the Kanin Peninsula (S e g e r s t r 11 I e 1948). temperatures during winter on all coasts, but the Finally Both G. setosus and G. oceanicus extend to difference is smallest along the east coast Spitsbergen and Novaya Zemlya (S tee I e and (S t e fan s son 1969). In summer, air temper­ S tee I e 1974). atures usually exceed sea temperatures for some With the exception of G. zaddachi, G. locusta and months, the difference being greatest along the M. marinus the above species also occ.ur on the 22 AGNAR INGOLFSSON

Table 7

The order in which 11 species of intertidal amphipods occurring in Iceland reach distributional limits towards a colder temperature regime (see text for literature sources).

Europe Iceland America (east coast)

1. Orchestia gammarellus 1. O. gammarellus 1. M. stoerensis 2. Marinogammarus marinus 2. M. marinus 2-3. O. gammarellus 3. Gammarus locusta 3. G. locusta M. finmarchicus 4. M. stoerensis 4. M. obtusatus 5. Hyale nilssoni 5. H. nilssoni 6- 7. M.obtusatus 6. G. duebeni M. finmarchicus 7. G.oceanicus 8- 9. G. zaddachi 8. G. setosus G. duebeni 10- 11. G. oceanicus G. setosus

American shore of the Atlantic. Going north­ may be more affected by air temperatures than wards, M. stoerensis appears to reach its limit first, other species here considered. at Nova Scotia (S tee I e and S tee I e 1974). O. These considerations lead to the general con­ gammarellus and M. finmarchicus reach southern clusion that temperatures are somehow control­ and eastern Newfoundland (B 0 u s fie I d 1973, ling the extension of the amphipods into cold S tee I e and S tee Ie 1974) while M. obtusatus temperature regimes. The way in which this is extends to northern Newfoundland (S tee I e achieved is undoubtedly so complex (S 0 u t h­ and S tee I e 1974). The ranges of H. nilssoni and war d 1958, Sou t h war d and C r i s p 1954, G. duebeni extend to southern Labrador, while the C r i s p and Sou t h war d 1958) that it may latter species also occurs in Greenland (B 0 u­ seem futile at this point to speculate further. It sfield 1973, Steele and Steele 1974). may be said, however, that in the present cases Finally G. oceanicus extends to northern Hudson there seems to be a closer correspondence of dis­ Bay and western Greenland (ca 74 ON) and G. tributionallimits with summer sea temperatures setosus goes all the way to northern Ellesmere (temperature data from Bah n c k e 1936 and Island (ca 83°N) (S tee I e and S tee I e 1974). K r a u s s 1958) rather than winter sea temper­ On the whole the species drop out in the same atures, although this corresspondance is certainly order on both sides of the Atlantic as well as not very close for some species, and there is slight around Iceland as one moves into colder temper­ indication that sea temperatures are more im­ ature regimes (Table 7). There appear to be two portant than air temperatures. exceptions, if adequacy of distributional data can None of the species considered reach dis­ be assumed. The two talitrid species, O. gam­ tributionallimits towards a warmer temperature marellus and H. nilssoni, extend considerably regime in Iceland, but G. setosus appears to come further into a cold temperature regime on the close to this. This is in agreement with its dis­ American side than on European coasts (includ­ tribution elsewhere where its southern limit is ing Iceland) relative to other species. This indi­ further northward than that of the other species. cates that distributional limits of these two In Norway the southernmost locality appears to species are controlled in a fashion different from be Rost (ca. 67 1/z0N) (Stephensen 1935­ that of other species, and it may be significant that 1942), while in America its southern limit lies in both species live under conditions where they Maine (S tee I e and S tee I e 1974). INTERTIDAL AMPHIPODS IN ICELAND 23

On the basis of the distributional patterns of more relevantly, in degree of movement of the these amphipods, therefore, the temperature substrate, in amount of silt, and there are un­ conditions in the intertidal region of Iceland doubtedly other unrecognized differences. The appear to be similar to those found on the amphipods show a spatially successional series o European Atlantic coast from about 67 1/2 N from sheltered shores with little movement of north to a region near the border of Norway and substrate and rich fucoid vegetation (type 5) to the Soviet Union. The corresponding area on the the exposed shores with considerable movement western side of the Atlantic is more difficult to of substrate and no fucoids (type 8) as follows (Fig. delimit, but appears to extend from Main north 4): M. obtusatus and M. ji"nmarchicus - Hyale nils­ to Newfoundland. soni and M. marinus - Gammarus oceanicus and The amphipod fauna of the intertidal zone of M. stoerensis - G. setosus. The overlap between Iceland is clearly European in character rather adjacent species on this series is great and most than American. In Norway there are several species occur on all types of shores to some degree. species of intertidal amphipods of the families It is not possible at the present time to correlate Gammaridae and Talitridae not found in Ice­ these differences among species with differences land. Among these, Gammarus salinus Spooner, in their adaptations, and the elucidation of fac­ Marinogammarus pirloti Sexton and Spooner, Tali~ tors controlling habitat partitioning along this trus saltator (Montagu) and Talorchestia brito Steb­ series will have to wait future studies. bing do not reach as far northwards as does O. Within each type of rocky shore the defined gammarellus (S t e p hen sen 1935-1942, subhabitats differ in tidal levels, which in turn Va d e r 1969, 1970, 1972). These species are results in differences in species composition, therefore not to be expected in Iceland where O. biomass and cover of algae, in degree of exposure gammarellus is only able to survive on the warmest to air, fresh-water and sea-water, in temperature coasts. Hyale pontica Rathke has on the other hand regimes, and in other ways. The amphipod been found in Norway north to about 70 0 N species show more or less distinct preferences for (V a d e r 1971), i.e. considerably further north particular tidal levels (Fig. 5), and they tend to than O. gammarellus but not as far as M. marinus. occur in the following order, moving downwards The discovery of this species on the shores of on the shore: O. gammarellus - G. duebeni - M. southwestern Iceland is therefore to be expected marinus or G. setosus and H. nilssoni - M. obtusatus in future studies. and M. finmarchicus - G. oceanicus. The vertical overlap between adjacent species is great. G. Habitat preferences duebeni is known to be able to survive in saturated The results show that each of the 11 species of air better than G. oceanicus (L age r s pet z 1963) intertidal amphipods here considered have their and O. gammarellus can survive considerable particular habitat distribution different from periods in saturated air (W i 11 i a m son 1951), that of other species, with the exception of M. which agrees with the usual high position on obtusatus and M. finmarchicus which appear very general seashores of these two species. In addition similar. All species show considerable overlap it seems likely that the tolerance of G. duebeni, G. with at least one another species in habitat setosus and M. marinus to low salinities (see below) preferences, and few habitats are inhabited by enables them to inhabit successfully high shore only one species (mostly involving G. duebeni). levels, where the effect of fresh-water run-off and The habitat categories used in this study differ precipitation can be expected to be considerable. from each other in a number of recognizable The habitats at different points in river chan­ ways. The four types of rocky seashores (habitat nels in estuaries differ primarily in salinity categories 5- 8) differ in fucoid species com­ regimes, and the amphipods tend to occur in the position, in fucoid cover, in total algal biomass, in following order towards the sea, again with large degree of exposure to wave action, or possibly overlap between adjacent species: G. duebeni - G. 24 AGNAR INGOLFSSON zaddachi - G. oceanicus and G. setosus. In the in­ occurs III a narrower range of habitats on the tertidal region to each side of the river channel relatively warm southern.and western coasts than the difference among points at similar tidal levels in other parts. While the temperatures of the along the estuary is presumably also primarily of ponds and pools that the species especially in­ salinity regimes, and here the order, going habits in the south and west may become con­ seawards, is usually as follows: G. duebeni - G. siderable higher than those of the sea on warm oceanicus and G. setosus - (M. stoerensis and M. sunny days, the temperatures on the general finmarchicus). Experiments have shown (B e a die seashore may be expected to become higher still and era g g 1940, Van Mar en 1975, D. H. under such conditions at low tide (S 0 u t h­ S tee I e and V. J. S tee I e 1970, V. J. war d 1958), so as to make this habitat unin­ S tee I e and D. H. S tee I e 1970, S u t­ habitable for the species. It is perhaps significant c I iff e 1968, We r n t s 1963, and own unpu­ that G. setosus occurs on exposed shores in blished data) that G. duebeni and G. zaddachi southern and western Iceland, where sea-spray tolerate reduce salinities or fresh water better may reduce the warming up of the shore at low than other Gammaridae species here considered, tide in sunny weather, whereas it is almost totally G. setosus and M. marinus come next, while G. lacking from sheltered shores here. oceanicus and M. finmarchicus are considerably less From the above it is clear that a number of tolerant. Finally M. obtusatus is relatively intoler­ interacting factors are controlling habitat selec­ ant of low salinities, although still a euryhaline tion of intertidal amphipods. Some of these fac­ species (no information is available on M. stoeren­ tors can be identified, others not. Apart from sis). The distribution of species in estuaries is in physical factors, biotic factors can also be ex­ good agreement with the differences in adap­ pected to be of importance, especially inter­ tations to low salinities but other factors such as specific competition, which might enhance tolerance to desiccation are evidently involved differences in habitat selection among species. also. In lagoons and coastal ponds species tend to There is, however, little evidence for this offered occur in a similar order as in river channels, by the present data. In particular, the absence of moving from lower to higher salinities. The ab­ M. marin us from northern and eastern coasts does sence of G. zaddachi from many small coastal not affect the vertical distribution of such species ponds and from tidepools (p. 9.) is rather puz­ as M. obtusatus, M. finmarchicus and G. oceanicus. In zling in view of its high tolerance to low salinities. the Netherlands M. marinus extends to lower levels It seems probable that low oxygen levels of these on the shore when M. obtusatus is absent, while habitats, produced by rotting algae, are respons­ there is a sharp boundary between the two when ible for the absence of G. zaddachi. G. duebeni which both are present (D e n H art 0 g 1964, V a d e r thrives in these habitats is known to be consider­ 1965). Den H art 0 g (1964) believes this to be ably more resistant to low oxygen levels than G. due to interspecific competition. Goo d h art zaddachi (Segerstrale 1946, Suoma­ (1941) also states that the two are never found I a i n e Ii 1958). together, although they may occur at similar Temperature is clearly an important factor tidal levels, while Bra t t e g ar d (1966), controlling macrodistributional patterns of in­ Pin k s t e rand S t 0 c k (1970), and Van tertidal amphipods but its role on a microdis­ Mar en (1975) have often found the two tributional scale is more difficult to evaluate. On together. Van Mar e n (1975) also found that the western coast of the Atlantic G. setosus with­ M. marin us did not extend further down on the draws to the outlets of cool fresh-water streams in shore than usual south of the range of M. obtusatus, summer towards it southern limit (V. J. and she believes, as did Va d e r (1965), that S tee I e and D.H.S tee I e 1970), presumably habitat differences between the two where the because temperatures become too high for it result of preferences for different substrates, M. elsewhere. In Iceland also, this arctic species marinus preferring more muddy substrates than INTERTIDAL AMPHIPODS IN ICELAND 25

M. obtusatus. No difference in this respect can be eastern coasts as compared to what obtains discerned in the Icelandic data (if anything, M. elsewhere. obtusatus has been recorded more often from mud thanM. marinus) but neither is there an indication of strong competition between them there. Although the two prefer different tidal levels, the ACKNOWLEDGEMENTS overlap is great. Of the total of 86 samples from Iceland containing M. marinus, M. obtusatus was I am grateful to Kristin Ada.lsteinsdottir M.Sc., thus present in no less than 40. Helgi Gudmundsson B.Sc., Erlingur Hauksson M. marinus and G. setosus occupy very similar B.Sc. and Dr. W. Vader for permission to use levels on the shore in Iceland and may occur on unpublished data. Drs. A]. and E.C. Southward similar shores (Fig. 5) but the two were only read a draft of a portion of this paper and made found together in two samples of the total of 95 helpful suggestions. I am grateful to my wife, containing either species from within the dis­ Linda Wendel, for much assistance in data ana­ tributional area of M. marinus. As the temperature lysis. Dr. W. Vader kindly checked the identifi­ regime of the warm shores of southern and wes­ cation of Gammaridae species and of Orchestia tern Iceland is probably getting close to the limit gammarellus. Many other people, too numerous to that G. setosus can tolerate, it may not be able to mention, have assisted in collecting data. This withstand competition from M. marinus here and paper was written while I was a visitor at the may only be able to succeed in habitats where the Laboratory of the British Marine Biological latter species is rare or absent (exposed shores, Association, Plymouth, at which time I was the coastal ponds, tidepools). it is worth keeping in recipient of a Nato Science Fellowship. mind in this connection that the two species could possibly replace each other geographically as competitors of o~her species, and if so the effects of competition might be difficult to detect REFERENCES by comparisons of habitat partitioning among Ad e y, W. H. 1968. The distribution of crustose species within and outside the distributional area Corallines on the Icelandic coast. Scientia Islan­ of M. marinus. dica, 1: 16-25. Be a die, L. C. and C rag g, J. B. 1940. Studies No significant differences were found between on adaptation to salinity in Gammarus sp. Regu­ habitat preferences of M. obtusatus and M. fin­ lation of blood and tissues and the problem of marchicus in Iceland, and there is a high degree of adaptation to fresh water. J. expo Bio!., 17: 153­ positive correlation between their occurrances in 163. samples. The two are known to differ in their B 0 u s fie I d, E. L. 1973. Shallow-water gam­ maridean Amphipoda of New England. Ithaca, tolerance to reduced salinities, and it seems likely Cornell Univ. Press. that an investigation on a finer scale than here Bra t t ega r d , T . 1966. The natural history of the used would reveal differences between them, Hardangerfjord. Horizontal distribution of the perhaps related to the microdistribution of salin­ fauna of rocky shores. Sarsia, 22: 1-54. ities on the shore at low tide. B rig g s, j . C. 1974. Marine zoogeography, New York, McCraw-Hill Book Co. The shores of southern and western Iceland Bah n e c k e, G. 1936. Atlas zu Temperatur, Salz­ differ from those of northern and eastern Iceland gehalt und Dichte an der Oberfliiche des Atlan­ in tidal range, this being about 3 - 4.5 m at spring tischen Ozeans. Wiss. Ergeb. d. Dtsch. At!. Exp. "Meteor" 1925-1927,5:1-VII. tides in the former area and 1-2 m in the latter. C h e v r e u x, E. and Fag e ,L .1925. Amphipodes. The only recognizable effect of tidal range on Fauna Fr., No.9, 488 pp. amphipods is a reduction in the vertical range of C r i s p , D .J. and Sou t h war d , A .J. 1958. The preferred areas of each species on northern and distribution of intertidal organisms along the 26 AGNAR INGOLFSSON

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